1. Field of the Invention
The present invention relates to a roller, a belt unit, and an image forming apparatus that uses a roller and a belt unit.
2. Description of the Related Art
Among conventional image forming apparatus are electrophotographic printers and copying machines. Such apparatus employ a transfer belt that transports a print medium such as print paper or an intermediate transfer belt via which a toner image formed on a photoconductive drum is transferred onto a print medium. These transfer belts are mounted on a plurality of rollers and run about the rollers.
The belt unit includes a drive roller and a driven roller, and the transfer belt is entrained about the drive roller and the driven roller with tension. A drive source such as an electric motor drives the drive roller into rotation, which in turn causes the transfer belt to run. When the drive roller rotates, the transfer belt runs about the drive roller and driven roller.
The driven roller is made of an electrically conductive metal material such as aluminum, machined into a specific shape and a size. A metal shaft is inserted into the driven roller. The metal shaft is supported at its longitudinal ends on bearings, so that when the transfer belt runs, the driven roller rotates on the metal shaft smoothly.
However, because the driven roller of the belt unit of the image forming apparatus is formed of an electrically conductive metal material such as aluminum, the belt unit tends to be heavy.
In addition, it is rather costly to machine a metal material into a desired shape and a size of a driven roller. One way of saving the cost of material of a driven roller and machining the driven roller is to use a mold for forming the driven roller.
FIG. 19 illustrates a conventional driven roller.
A driven roller 101 is in the shape of a long hollow cylinder having a longitudinally extending through-hole 102. Thus, when the driven roller 101 is to be formed by using a mold, the mold is required to have a long projection corresponding to the through-hole 102. The driven roller 101 is taken out from the mold in a direction shown by arrow A. The long projection is tapered such that the driven roller 101 can be taken out smoothly from the mold. That is, one end of the through-hole 102 has a diameter D1 and the other end has a diameter D2 smaller than D1.
The metal shaft has the same outer diameter across its length and therefore the diameter D1 makes a larger gap between the shaft and the driven roller than the diameter D2. As a result, the circumferential speed of the outer surface of the driven roller varies across the length of the driven roller, causing the transfer belt to snake or tend to displace to one side of the driven roller.
FIG. 20 illustrates, with some exaggeration for explanation, when there is a gap between shaft and the inner surface that defines the through-hole 102. The gap varies along the length of the roller. When the transfer belt runs, the gap causes the transfer belt to snake or shift toward one longitudinal end of the roller.